1. Technical Field
This disclosure relates generally to liquid discharge heads, liquid dischargers, and image forming apparatuses, and more particularly to a liquid discharge head having a water-repellent layer at a surface, a liquid discharger including the liquid discharge head, and an image forming apparatus including the liquid discharge head.
2. Description of the Related Art
Some of the image forming apparatuses such as printers, facsimile machines, copiers, and those having two or more of the functions of these apparatuses perform image forming (recording or printing) by causing recording liquid (hereinafter also referred to as “ink”) as liquid to adhere to a medium (hereinafter also referred to as “paper” or “paper sheet,” but not limited to paper in material; “medium to be subjected to recording,” “recording medium,” “transfer material,” and “recording paper” may also be used as synonyms) while conveying the paper, using, for example, a liquid discharger (liquid discharge device) including a recording head formed of a liquid discharge head that discharges liquid droplets of the recording liquid.
The term “image forming apparatus” means an apparatus that performs image forming by discharging liquid onto media such as paper, thread, textile, cloth, leather, metal, plastic, glass, wood, and ceramics. The term “image forming” means not only providing media with significant images such as letters, characters, and figures, but also providing media with insignificant images such as patterns. Further, the term “liquid” is not limited to recording liquid and ink, and may be any liquid as long as images can be formed with it. Further, the term “liquid discharger” means an apparatus that discharges liquid from a liquid discharge head.
The liquid discharge head performs recording by causing liquid droplets to be discharged from nozzles. Accordingly, the nozzle shape and accuracy greatly affect its ink droplet ejection characteristics. Further, it is known that the surface characteristics of a nozzle formation member in which nozzle holes are formed also affect the ink droplet ejection characteristics of the head. For example, it is known that if ink adheres to the fringe of a nozzle hole on the surface of the nozzle formation member so that a non-uniform ink pool is formed, various problems are caused. For example, ink droplets are discharged in curved directions, variations are caused in ink droplet size, and the flying velocity of ink droplets becomes unstable.
Therefore, in common liquid discharge heads, an ink-repellent layer (film) or a water-repellent layer is formed at the surface on the liquid droplet discharging side. For example, in addition to forming a fluorine-based water-repellent film as a water-repellent layer, forming an ink-repellent layer of a compound having a perfluoropolyether chain and an alkoxysilane residue is disclosed in Japanese Laid-Open Patent Application No. 2003-019803, forming a water-repellent film of a fluororesin polymerization film or a silicone resin polymerization film is disclosed in Japanese Patent No. 3755647 (corresponding to Japanese Laid-Open Patent Application No. 2003-72086), and forming a water-repellent film of a silicone resin layer by electrostatic painting is disclosed in Japanese Laid-Open Patent Application No. 2005-138383.
Further, as a method of forming a water-repellent film, Japanese Patent No. 3379119 discloses applying a water and oil repellent film on a nozzle surface by dipping the nozzle surface in a solution in which a water and oil repellent material is dissolved while keeping gas emitted from the nozzle bottom side to the nozzle surface; thereafter removing the nozzle surface from the solution while emitting the gas; and then leaving the nozzle surface.
In recent image forming apparatuses, various modifications have been made to the composition of recording liquid (ink) in order to form a high-quality image on plain paper in particular. Characteristics required as recording liquid include color tone, image density, and bleeding for achieving high quality; the dissolution or dispersion stability of a coloring agent in recording liquid, storage (preservation) stability, and discharge stability for achieving reliability; water resistance and light fastness for ensuring the keeping quality of a recorded image; and the quick drying of recording liquid for achieving high speed.
For example, at the beginning, dye inks using dye as a coloring agent were the mainstream of recording liquid because of their good color development and high reliability. In these years, however, ink compositions using pigment such as carbon black have been used in order to provide recorded images with light fastness and water resistance. Further, some recording liquids use fluorochemical (a fluorochemical or fluorine-containing surfactant) as a surfactant added as a recording liquid component. Further, in terms of the viscosity of recording liquid, it is necessary to use highly viscous ink in order to ensure high-speed printing on plain paper with high printing quality.
Specifically, with respect to conventional liquid discharge heads using fluororesin for a water-repellent layer, a method that forms a PTFE thin film on the surface of a metal nozzle plate by Ni-PTFE eutectoid plating and heat treatment thereon and a method that coats the surface of a metal or resin nozzle plate (base material) with a fluorochemical water-repellent agent are adopted.
Such nozzle formation members using fluororesin for a water-repellent layer can have good water repellency (ink repellency) with respect to known dye inks and pigment inks (using pigment as coloring material) having surface tensions exceeding 30 mN/m.
According to experiments conducted by the inventor of the present invention, however, it has been confirmed that there is a problem in that sufficient water repellency (ink repellency) does not appear in the case of using inks having a low surface tension of 30 mN/m or less and, in particular, in the case of using inks having fluorochemical added thereto.
As a result of studies for improving water repellency to inks having a low surface tension of 30 mN/m or less and inks having fluorochemical added thereto, it has been found that a silicone resin coating is effective as a water-repellent layer as is also disclosed in Japanese Laid-Open Patent Application No. 2005-138383 described above. It has also been found, however, that sufficient water repellency cannot be produced with respect to inks having low surface tensions, in particular those of a low surface tension containing a fluorochemical surfactant, by merely forming a silicone resin layer.